The invention pertains to the field of detecting and discriminating between small concentrations of atomic hydrogen and atomic deuterium.
The properties of atomic hydrogen and deuterium are of fundamental interest for atomic physics, astrophysics, chemistry, surface physics, and plasma physics. Current efforts to achieve controlled fusion both by magnetic and inertial confinement require techniques for studying and monitoring the characteristics of plasmas and in particular, hydrogen plasmas. There is great interest in being able to detect small concentrations of atomic hydrogen in a vacuum or in the presence of a background gas, in being able to detect small concentrations of atomic deuterium in a vacuum or in the presence of a background gas, and in being able to discriminate between the two. An application of laser techniques in devising methods for performing these tasks is found in "Doppler-Free Two-Photon Spectroscopy of Hydrogen 1S.fwdarw.2S" by T. W. Hansch, S. A. Lee, R. Wallenstein and C. Wieman, Physical Review Letters, Vol. 34, No. 6, Feb. 10, 1975, pp. 307-309. The article discloses an apparatus for performing an experiment to study the 1S.fwdarw.2S transition in atomic hydrogen and deuterium by Doppler-free two-photon spectroscopy using a frequency-doubled pulsed dye-laser at 2430 A. The atoms were excited by absorption of two photons of wavelength 2430 A and the excitation was monitored by observing the subsequent collision-induced 2P.fwdarw.1S fluorescence at the L.sub..alpha. wavelength 1215 A. The gas was exposed to counter-propagating beams. The experiment discloses a problem in that the L.sub..alpha. line signal is reduced due to resonant absorption by other hydrogen or deuterium atoms. This required a small separation between the illuminated region of the gas and the L.sub..alpha. detection windows. Filters are also required to reduce the off-resonance background signals. A further problem results from the use of counter-propagating laser beams. This causes L.sub.60 to be generated along the entire region of exposure and does not allow one to map the spatial distribution of the hydrogen or deuterium in three dimensions. Thus, this technique suffers because the resonant L.sub..alpha. emission is self-trapped or may be absorbed by a background buffer gas. This makes efficient detection of the excitation difficult. This type of emission spectroscopy as applied to plasmas consists of the passive monitoring of side light from the plasma. Consequently, the spatial and temporal resolution is poor. In an optically thick plasma, only the outer sheath of the plasma can be studied. For plasma constituents whose ground state transitions lie in the VUV, monitoring emission from the first excited state is complicated by the special optics required. Finally, the ground state density cannot be determined directly by emission measurements.
Multiphoton-ionization spectroscopy has been used as a tool for spectroscopic investigations of atoms and molecules. In particular, this is discussed in "Multiphoton Excitation and Ionization of Atomic Cesium with a Tunable Dye-Laser" by D. Popescu, C. B. Collins, B. W. Johnson and I. Popescu, Physical Review A, Vol. 9, No. 3, March 1976, pp. 1182-1187. The article discloses an apparatus for performing multiphoton ionization spectroscopy in atomic cesium. It discusses three-photon ionization of cesium as a method by which specific two-photon electronic transitions may be studied. The method involves the use of a single frequency tunable laser whose frequency lies near a single photon resonance in cesium to provide an enhanced two-photon excitation. This method of utilizing a laser frequency which is nearly resonant with an intermediate single photon transition is inappropriate in hydrogen or deuterium because there is no such convenient state lying between the 1S and 2S states in hydrogen. The method of using a single laser frequency causes ions to be generated along the entire path of the laser beam through the gas and will not enable a three dimensional spatial mapping of the distribution of atoms to be made.
Monitoring of ground state densities of plasma constituents by monitoring fluorescence after single photon resonant excitation is complicated by the difficulty of generating coherent photons for transitions in the VUV, and cannot be used to probe within optically dense plasmas. A further difficulty is the poor signal-to-noise ratio obtained from a weak optical signal (fluorescence) which is detected against the strong emission background of the plasma.
Lastly, optogalvanic spectroscopy for determining the properties of plasma constituents involves the measurement of the change in the voltage drop across a plasma due to the single photon resonant excitation of a plasma constituent. Because this technique relies on ionization by electron collision subsequent to the absorption of a photon, the voltage changes are small, typically less than 1 percent of the total drop. This limits the dynamic range and resolution of the technique. Furthermore, optogalvanic spectroscopy suffers from the same drawbacks as resonant fluorescence in exciting VUV ground state transitions. And lastly, this technique does not provide a method for obtaining a three-dimensional spatial map of an optically dense plasma.